Long-distance migrants are assumed to be more time-limited during the pre-breeding season compared to the post-breeding season. Although breeding-related time constraints may be absent post-breeding, additional factors such as predation risk could lead to time constraints that were previously underestimated. By using an automated radio telemetry system, we compared pre- and post-breeding movements of long-distance migrant shorebirds on a continent-wide scale. From 2014 to 2016, we deployed radio transmitters on 1,937 individuals of 4 shorebird species at 13 sites distributed across North America. Following theoretical predictions, all species migrated faster during the pre-breeding season, compared to the post-breeding season. These differences in migration speed between seasons were attributable primarily to longer stopover durations in the post-breeding season. In contrast, and counter to our expectations, all species had higher airspeeds during the post-breeding season, even after accounting for seasonal differences in wind. Arriving at the breeding grounds in good body condition is beneficial for survival and reproductive success and this energetic constraint might explain why airspeeds are not maximised in the pre-breeding season. We show that the higher airspeeds in the post-breeding season precede a wave of avian predators, which could suggest that migrant shorebirds show predation-minimizing behaviour during the post-breeding season. Our results reaffirm the important role of time constraints during northward migration and suggest that both energy and predation-risk constrain migratory behaviour during the post-breeding season.
Although within‐year site fidelity to specific wintering sites allows shorebirds to use prior knowledge of resources and microhabitats, such fidelity may also make populations more vulnerable to extirpation in the event of increased predation pressure, habitat loss, or disturbance. In the eastern Atlantic, Purple Sandpipers (Calidris maritima) have been found to be highly faithful to specific sites in wintering areas. However, little is known about the use of wintering areas by these sandpipers along the coast of Maine. We quantified movements of 60 radio‐marked Purple Sandpipers in a bay near the mainland and on an offshore cluster of islands along the mid‐coast of Maine during two winters (2005–2006 and 2006–2007). Birds marked in early‐ and mid‐December remained until spring migration, with no evidence of onward migration. Mean maximum distances moved did not differ significantly between either males (8.6 ± 1.0 [SE] km; N= 30) and females (7.4 ± 0.8 km; N= 30) or juveniles (9.9 ± 1.6 km; N= 9) and adults (7.8 ± 1.1 km; N= 26). We also detected no monthly (January–May) differences in maximum distances moved. Sixty percent of marked individuals moved ≤5 km between the two most distant relocations and no birds moved >25 km during the 2‐ to 4‐month tracking period. We attribute the high site fidelity primarily to the plentiful prey base in the study area. During a 2‐d period with severe cold, feeding areas at locations protected from wave action became encased in ice and birds at these locations moved up to 10 km offshore to sites with less ice. Species with strong site fidelity, like wintering Purple Sandpipers, may be at higher risk in the event of large‐scale changes in their food base, increased predation pressure, habitat loss, or disturbance. However, the short‐distance movements made when intertidal feeding areas became encased in ice suggest that Purple Sandpipers could potentially move greater distances in response to changing conditions in their wintering areas.
to a greater understanding of stopover behavior and the degree of connectivity among stopover sites, which should be taken into account for conserving migratory bird habitat across broad spatial scales.
The Purple Sandpiper (Calidris maritima) is a medium‐sized shorebird that breeds in the Arctic and winters along northern Atlantic coastlines. Migration routes and affiliations between breeding grounds and wintering grounds are incompletely understood. Some populations appear to be declining, and future management policies for this species will benefit from understanding their migration patterns. This study used two mitochondrial DNA markers and 10 microsatellite loci to analyze current population structure and historical demographic trends. Samples were obtained from breeding locations in Nunavut (Canada), Iceland, and Svalbard (Norway) and from wintering locations along the coast of Maine (USA), Nova Scotia, New Brunswick, and Newfoundland (Canada), and Scotland (UK). Mitochondrial haplotypes displayed low genetic diversity, and a shallow phylogeny indicating recent divergence. With the exception of the two Canadian breeding populations from Nunavut, there was significant genetic differentiation among samples from all breeding locations; however, none of the breeding populations was a monophyletic group. We also found differentiation between both Iceland and Svalbard breeding populations and North American wintering populations. This pattern of divergence is consistent with a previously proposed migratory pathway between Canadian breeding locations and wintering grounds in the United Kingdom, but argues against migration between breeding grounds in Iceland and Svalbard and wintering grounds in North America. Breeding birds from Svalbard also showed a genetic signature intermediate between Canadian breeders and Icelandic breeders. Our results extend current knowledge of Purple Sandpiper population genetic structure and present new information regarding migration routes to wintering grounds in North America.
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